Ok, my background is army, but I'm going to post this assuming the branches work similarly.

Any type of school is not a PCS, but rather a TDY. For TDY, the military will typically not pay for moving household goods, moving family, or for a place for the service member and family to stay. Typically, sm will be required to stay in some type of barracks where there will not be room for children, and even if there were, children are not allowed. The sm may assume these costs (moving, housing), but it is at personal expense.

Firstly, if she's so poor she can't afford mediation, how will she shoulder all this? Does she expect your DH to pay half (or all)?
Secondly, I have to agree w/ HRNYC. I understand that there are state standards, and I lived for quite a while in TX near Hood. Schools in military communities are aware of the difficulties faced by military families in terms of moves and often offer them an early out alternative that can be as much as one month (though usually is 2 weeks). The school districts in TX *do* offer this option. The school time missed as a result of taking the early out does not count against absences. So there you go! Relax. Have a mimosa. The 6 y.o. will not be held back.

The BM first responsibility is to attend this in-residence school and adhere to the military requirements for attendance. If the military school states that dependents are NOT authorized, then she may be ASSUMING/HOPING she can get temporary family lodging on base for 8-9 months - which will NOT be the case. Furthermore, if she thinks her mother is going to care for the children, then she has to have something official in place for that as well. Her mother just can't come and go as she pleases on base without an I.D. card. And mom can't get an I.D. card unless she is identified as an official "dependent", too. So the BM's better be financially prepared for an alternate plan that she may have to pay out of her pocket for an apartment for her, the kids and her mom. Seems to me that BM is making a lot of assumptions as to what military benefits she is authorized at this school. First thing I would do is have your DH ask for verification (a copy of her military orders) which state whether or not dependents are authorized while she is in school. If dependends are not authorized, the military has made it thus for good reason (demands of the school, lack of free time, etc.) and I would certainly bring that up as a basis for keeping kids with dad. Plus, all military members must have a dependent care plan in place for when they are deployed, etc. I would imagine that unless otherwise proscribed by law, your DH (the father) should be listed on that plan - not the BM's mother.

Even if the school DOES authorize dependents, then the issue of her mom gaining on-off base access without an I.D. is still a hurdle - which would still put them back outside the gate looking at renting an apartment. And some military schools do not permit that, either, requiring students to live on-base. But the copy of her orders would detail that.

If the divorce agreement states you can review visitation depending on her orders/reassignment/duty status, then I would say this is absolutely an occassion to do just that - and do it with full information from the military as to the type of school and whether she is authorized dependents, and the full authority of court.

With a little sleuthing of your own, you may be able to find out based on the little she's given you. If you know the specific TX military installation and what kind of school she may be attending (based on her military career -- medical, administrative, vehicle maintenance, NCO training, etc.) that would help.

You can contact the military base in TX (via their website or the "contact number" usually the public affairs office) and ask the following, "There is a course for XXX on your base on these dates. Can you please tell me if accompanied dependents are authorized for this particular training course?" See what they say! It never hurts to ask. Of course, I would have DH do the inquiring as he has a right to know as the kids bio-father.

In order for an adult to be a dependent, they have to be exactly that. I believe the mother would have to prove that she could not financially or physically sustain herself. The military doesn't just add adults as dependents (other than spouses) willy-nilly - there has to be some significant, sustained reason - and believe me, it would take months and REAMS of documentation and paperwork to do that!

Schools are a PCS if they're a certain length. I PCSed to both of my training schools, (one 63 weeks and the other 20 weeks) and DH just went through a school PCS. (03 Aug 10 - 17 Jun 11) I sincerely doubt that your BM's school is the same one he went to (Ft Bliss/El Paso).

My MOS being what it is, I automatically think she's retraining into one of my MOSes, since my branch is HUGE in MD - but she could be medical, (San Antonio?) and that actually makes a lot of sense... What city is BM going to?

Have your DH threaten an ex parte motion if BM won't go to mediation. The girls have a stable life where they are and ripping them away, especially when BM will be so busy with school... your DH is just as if not more stable than she is. Judges, as a rule, don't like active duty military anyway. (so say our personal experiences)